1. Field of the Invention
The invention relates to a backwash valve, for use in a sucker-rod actuated fluid pump disposed in a length of production tubing, which permits the pump and length of tubing to be drained.
2. Description of the Prior Art
A conventional oil well includes a cased well bore with one or more strings of tubing extending downwardly through casing into the oil or other petroleum fluid contained in the sub-surface mineral formation to be produced. The casing is perforated at the level of the production zone to permit fluid flow from the formation into the casing, and the lower end of the tubing string is generally open to provide entry for the fluid into the tubing.
One type of pump conventionally employed in structures of the type described is wedged into an internal constriction, or seating nipple, formed internally of the production tubing below the fluid level. A metallic enlargement on the external body of the pump prevents it from traveling below the seating nipple, and resilient seal rings on the body of the pump housing, or pump barrel, act to form a leak proof seal between the seating nipple and the pump housing, or barrel. Typically, beneath the seating nipple there are disposed a conventional gas anchor and mud anchor, the mud anchor having a perforated outer surface, through which the petroleum fluid may pass into the production tubing. The pump is generally driven by a mechanical linkage of metal rods, generally referred to as sucker-rods, or valve rods, which extend from the pump to the well surface. The valve rod, or sucker-rod, linkage is powered in a reciprocating motion by conventional mechanical apparatus, usually called a pumping unit, located at the well surface.
The conventional pump generally includes a housing through which a piston is reciprocated by the sucker-rod, or valve rod, linkage. In its simplest form, the conventional pump of the type described often includes a number of ball and seat valves with one such valve in the lower end of the moveable piston, and another at the inlet port of the housing or pump barrel. On the upstroke of the plunger, or moveable piston, the ball in the inlet port valve, or lower standing valve, is drawn away from its seat and the ball of the outlet port valve, or traveling valve, is forced over its seat to draw fluid from below the seating nipple and into the housing. On the piston's down stroke, the ball in the inlet valve, or standing valve, is forced onto its seat and the ball in the piston valve, or traveling valve, moves away from its seat to allow the piston, or plunger, to move downwardly through the fluid contained in the housing, or pump barrel. On the subsequent upstroke, the closing of the piston valve, or traveling valve, forces the fluid above the piston, or plunger out of the housing, or pump barrel, through its outlet ports and into the production tubing string above the pump and simultaneously fills the pump barrel below the piston with fluid. Repetition of this cycle eventually fills the production tubing, or tubing string, and causes the fluid to flow to the surface.
The previously described pump, or some variation thereof, is probably the most widely employed in applications where it is desired to drive a sub-surface pump by a surface powered, mechanical linkage. A significant problem in pumps of this type is that paraffin and other contaminants can adhere to the interior surface of the production tubing string, the paraffin and other contaminants being found in the petroleum fluid being pumped from the well. As the paraffin, and other materials, adhere to the interior surface of the production tubing, as well as the sucker-rod linkage, these materials can eventually decrease the amount of desired fluid which can be pumped through the tubing, because the paraffin coating can eventually, substantially block the passage of fluid upwardly through the production tubing string. In order to remove the undesired paraffin coating, it is then necessary to "hot oil" the well to remove the undesired paraffin coating. When a well is hot oiled, a quantity of oil, which has previously been produced from the well and is stored in a holding tank at the earth's surface, is heated and poured into the well through the annular space between the casing string and production tubing string. The quantity of heated oil, typically 60-70 barrels of heated oil, then passes down the annular space between the casing string and production tubing string, until it is in the well below the seating nipple in the production tubing string, at which time the heated oil is drawn upwardly into the production tubing string by the action of the pump. As the heated oil is pumped upwardly through the production tubing string, it melts the paraffin buildup within the production tubing string so that it will be pumped upwardly to the earth's surface and reenter the holding tank.
A major problem with this hot oiling technique is that much of the heat contained within the heated oil is dissipated as it passes down the well to a level below the seating nipple. The heated oil may lose much of its heat to the casing string and the exterior surface of the production tubing string, whereby the heated oil, upon entering the pump and production tubing string may not be as hot as necessary, or desired, to efficiently melt the paraffin coating on the interior surface of the production tubing string. Another problem associated with this hot oil process is that the heated oil may also dislodge other undesired materials, which are typically referred to as "gyp" or "scale", disposed upon the inner surface of the casing string, and this material is likewise pumped through the pump and into the production tubing string. Such other undesired contaminants, such as gyp and scale, as well as sand from the well, can cause sealing problems with the ball and seat valves contained in the pump, as well as can cause undesired corrosion of the other pump parts and the sucker-rod linkage which reciprocates the pump. Additionally, this undesired material and paraffin, is in turn pumped back into the holding tank where it collects, which is undesired, and may very likely be repumped back into the well when the well needs to be hot oiled again. It would be a definite advantage if the production tubing string could be drained through the pump and the heated oil poured directly into the interior of the production tubing string at the earth's surface.
In a similar fashion, many wells are chemically treated, wherein it is desired to apply corrosion inhibitors to the components of the pump and the sucker-rod linkage. The corrosion inhibitors, which can be quite expensive, are likewise passed into the annular cavity between the casing string and production tubing string until they pass to a location within the well adjacent the seating nipple. The corrosion inhibitor is then pumped upwardly by the pump and through the production tubing string, at which time the corrosion inhibitor coats the pump components and the sucker-rod linkage. Much of the expensive corrosion inhibitor can be lost as it passes down the annular cavity between the production tubing string and the casing string, in that the corrosion inhibitor will likewise coat the interior of the casing string and the exterior surface of the production tubing string, which do not need to be treated with the corrosion inhibitor. Again, it would be advantageous if the production tubing string could be chemically treated by pouring the chemicals directly into a drained production tubing string.
Another problem associated with such pumps is that if it becomes necessary to remove the production tubing string and pump from the well, it is not possible to open the traveling valve and standing valve of the pump to permit the fluid in the production tubing string to be drained downwardly through, and outwardly from, the pump. Thus, the fluid which has been previously pumped and is contained in the production tubing string above the pump must remain in the production tubing while the production tubing string is being raised to the earth's surface. This is typically referred to as "pulling a wet string". The removal and handling of the fluid contained in the production tubing string can present environmental problems if such fluid is not properly handled and stored as the wet string is being pulled out of the well.
Accordingly, prior to the development of the present invention, there has been no backwash valve, for use in a sucker-rod actuated fluid pump disposed in a length of production tubing, which: permits the production tubing string and sucker-rod linkage to be hot oiled by directly passing the heated oil into the production tubing string; permits the production tubing string and sucker-rod linkage to be chemically treated with corrosion inhibitors by directly injecting the corrosion inhibitors into the production tubing string; and permits the production tubing string to be drained prior to pulling the production tubing string to the earth's surface. Therefore, the art has sought a backwash valve, for use in a sucker-rod actuated fluid pump disposed in a length of production tubing which: permits the production tubing string and sucker-rod linkage to be hot oiled by directly injecting the heated oil into an empty production tubing string; permits corrosion inhibitors to be directly injected into the production tubing string to treat the interior of the production tubing string and the sucker-rod linkage disposed therein; and permits the production tubing string to be drained prior to pulling the production tubing string from the well.